1. Field of the Invention
The present invention relates to the field of earth boring tools and more particularly to apparatus and methods for obtaining cores from formations within bore holes.
2. Description of the Prior Art
For many years the oil industry has suffered with poor coring performance resulting in high costs of obtaining a core from hard, fractured formations. The primary problem associated with these formations is frequent jamming inside the inner tube, in the bit throat and in the core catcher area. Jamming increases the number of trips required, increases damage to the core and can result in poor recovery due to the inability of the catcher to reliably grip and hold the broken core. Core from layered, fractured formations can be of interest to the geologists and reservoir analysts, but core recovered from these particular sections, utilizing conventional coring equipment, is frequently so badly damaged that many core analysis techniques cannot be accurately employed. Thus, the core, the acquisition for which a premium was paid, is of little value.
In response to these shortcomings, a novel core barrel has been developed by the assignee of the present invention wherein a rubber sleeve is provided for jacketing the core, see for example Austin "Core Barrel Apparatus" U.S. Pat. No. 3,012,622. Unfortunately, the rubber sleeve core barrel is unsatisfactory for coring hard, fractured formations as the sharp edges of the formations easily cut the rubber sleeve. In addition, a rubber sleeve cannot be used at high temperatures such as are encountered in deep or geothermal wells where hard, fractured rock is often encountered.
In response thereto, the assignee of the present invention developed a new core barrel wherein a specially designed wire mesh sleeve is employed. A woven or braided wire mesh sleeve constricts about the core when under tension, thereby grasping and lifting the core within the inner barrel. The wire mesh sleeve is pulled around the lower end of the inner tube into the core barrel at the same rate as the core is cut and acts as a cylindrical conveyor. It grips and supports the weight of the core, lifting it up the inner barrel, and serves as a continuous core catcher. When under tension, the wire mesh sleeve decreases in diameter and grips the core, keeping the core in its original diameter and thereby prevents jams. The same compacting force of the wire mesh sleeve when under tension also retains unconsolidated small pieces of rock in their original orientations, prevents relative movement of unconsolidated materials within the core and prevents additional core damage.
The improved core barrel sleeve as described in greater detail in co-pending applications entitled "A CORING DEVICE WITH AN IMPROVED CORE SLEEVE AND ANTI-GRIP COLLAR," Ser. No. 530,784, filed Sept. 9, 1983; "A CORING DEVICE WITH AN IMPROVED WEIGHTED CORE SLEEVE AND ANTI-GRIPPING COLLAR," Ser. No. 530,783, filed Sept. 9, 1983; and "A CORING DEVICE WITH AN IMPROVED CORE SLEEVE AND ANTI-GRIPPING COLLAR WITH A COLLECTIVE CORE CATCHER," Ser. No. 537,115, filed Sept. 29, 1983.
Although the wire mesh core barrel just described provides outstanding service in many applications, it remains subject to some operational disadvantages. Firstly, the weight on the drilling bit, that is the force which causes the bit to drill into the rock formation, is produced within such a prior system only by a limited pressure drop in the tool. The pressure drop acts across the sealed area of a slip joint which is used to tension the wire mesh core. In addition, the use of a slip joint can make core jams difficult to detect in some circumstances. In soft formations the rotary table mud pumps must first be stopped before the slip joint can be closed, thereby allowing an additional segment of the core to be cut while the core sleeve is maintained under tension. Stopping the rotary table mud pumps is not only a disruption to the drilling operation but can in some instances cause additional damage to the core or initiate a core jam. Finally, a core sleeve using a slip joint is particularly susceptible to being prematurely activated in an offshore floating drilling platform because of normal wave action. In such cases, the wave action may jack the stripper tube up prematurely.
Therefore, what is needed is an apparatus and methodology for lifting a stripper tube and sleeve within a core barrel as the bit penetrates the formation in a manner which is not subject to the above denoted difficulties.